Abstract

The Triassic Yanchang Formation lacustrine shale is a source of conventional oil accumulation in the Ordos Basin, China. The Yanchang Formation, a hybrid system of organic-rich shale, interbedded silty shale, and siltstone, is believed to be a potential unconventional oil and gas play. Our crossdisciplinary investigation of the storage space included the outcrop description, core observation, thin sections, and scanning electron microscope pore imaging. We evaluated the results from these techniques to reveal that the storage space within the Yanchang Formation shales included primary intergranular pores, secondary generated pores, tectonic fractures, and bedding-parallel fractures. We conducted adsorption experiments, combined with burial and thermal history, in which the primary migration process can be divided into three stages. In the Early Jurassic, organic matter did not reach the oil generation threshold. From the Late Jurassic to the Early Cretaceous, organic matter entered the oil generation window, and gas was generated and stored as adsorbed gas, dissolved gas, and free gas. From the Middle to Late Cretaceous, the storage of shale gas was dynamically transformed by tectonic uplift. Variations in chemical and carbon isotopic compositions from canister-core desorption were directly related to the gas supply in shales. An abrupt decrease in gas dryness and positive d13C1 values indicated the depletion of gas supply drainage. Our ultimate recovery factor reached 70%.

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